The use of diodes as microwave switches is well-known in the art and they offer a number of advantages over mechanical switches. There are no movable switch contacts with the attendant problem of noise caused by poor connections and the lower switch operating speeds caused by mechanical inertia. In addition, diode switches can be fabricated at a lower cost than comparable mechanical switches. Diode switches can also be packed in a smaller volume and, in particular, they are readily incorporated into transmission lines used to carry high-frequency energy.
The radio frequency power that can be controlled by a diode switch is determined by many factors well-known in the art. Several of these factors are:
The frequency of the microwave energy passing through the diode,
The speed at which the diodes are switched between its nonconducting and conducting states,
The frequency at which the diode is switched alternately between its conducting and non-conducting states,
Power losses within the switch and the voltage standing wave ratio in the switch caused by impedance mismatches therein.
In a typical diode microwave switch isolation in the order of 60 decibels (db) between the input and the output of the switch when the switch is non-conducting is required. In the art it is well known that a single diode can typically provide in the order of 25 db isolation. Accordingly, multiple diodes are used to achieve the desired isolation and, typically, 3 diodes are used to achieve 60 db isolation. Diodes may be connected in series with the transmission line conductors but they are typically connected in shunt across the transmission line conductors through the switch in order to achieve maximum power handling capability while achieving fast switching action. When such a switch is in its open or non-conducting state, all the shunt connected diodes are placed in a conducting state in a manner well known in the art. Vice versa, when such a switch is in a closed or conducting state, the shunt connected diodes are caused not to conduct. Microwave energy input to a shunt connected switch in its open or non-conducting state is reflected back to the input of the switch and, conversely, microwave energy input to a shunt connected switch in its closed or conducting state is coupled to the output of the switch. Such a shunt connected microwave diode switch is disclosed in U.S. Pat. No. 3,959,750, issued May 25, 1976, to Frank Holt.
In Microwave diode switches the conductors are typically mounted on dielectric material which is in turn mounted on or in close proximity to metallic material which is at ground potential. In addition, there are many connections between the small diode chips, the conductors interconnecting the diodes, capacitances, power sources and to connectors by which the microwave energy is input to and output from the switch. Each transition between differing materials such as between a connector and a conductor mounted on a dielectric substrate, between the conductor and the diodes, as well as between all the other elements typically found within a microwave diode switch normally cause some physical discontinuities or small impedance mismatches which result in power reflections that increase voltage standing wave ratios (VSWR) and dielectric losses within the switch. These are present problems in the microwave diode switch art.
To solve the above-described problems, I provide a novel combination of means for reducing dielectric losses and providing better impedance matches in the diode switch. More particularly, I mount diode chips on individual electrically conductive pedestals and the individual diodes are interconnected by conductors mounted on dielectric substrates which bridge the tops of the pedestals upon which each of the diodes is mounted. In addition, the dielectric mounted conductors are shaped to minimize or eliminate impedance mismatches between conductors and the conductive ribbons which connect the conductors to the diode chips on the pedestals.